I would not recommend going much over 1.2V. The 28nm Cape Verde core with Hynix memory IC's used on your card are rated to 1250 MHz, set at 0.825V to 1.16V factory settings. Power Saving Technology will drop down to 300 MHz core / 150 MHz memory @ 0.825V while in 2D / idle, and 1.16V load.
If you use MSI afterburner you should max out at 1125 MHz core and 1300 MHz memory, But.. HIS iTurbo overclocking software will go to 1145 MHz core and 1565 MHz memory.
These cards run great CrossFired @ factory settings.